That's because stupid, repressed, embarrassed parents cannot bring themselves to explain what the body consists of -- like eyes, nose, mouth, ears, brain, throat, tummy, willy, arms, hands, fingers, legs, feet, toes, ....and in mummy's case, a vagina and a W-O-M-B ! There! You've managed to get that awful word out! Wasn't so bad, was it? Dab cold water on your face if it is still bright red with shame.
Yeah, like confusing him or her with toilet training, not eating with grubby fingers, watching out for traffic, and so on.
Never heard of books, have you?
Oh, sure! Look at the vast armies of repressed and intimidated children out there who have correctly had explained to them where babies grow. I really don't know how primary school teachers cope with all the terrible repression and intimidation. It is simply shocking that it's allowed to go on.
MM